The invention relates to a layered composite on a substrate comprising at least one individual layer deposited on the substrate and including cubic boron nitride.
Because of its excellent material properties, cubic boron nitride is a super-hard material—after the diamond the hardest material—particularly suitable for use in connection with cutting tools such as chisels, cutting- or drilling tools as well as shaping tools in order to improve their service life and for high operating speeds. Cubic boron nitride even has a large advantage over diamonds in that it is chemically resistant with respect to iron materials.
The synthesis of thin cubic boron nitride layers, that is layers of a thickness of between 50 nm and 300 nm, is possible only by PVD or PECVD methods because during the formation of the super-hard cubic phase the layer must be subjected to ion bombardment. The ion bombardment, however, is accompanied by the development of such high internal stresses that the layers, beginning with a certain thickness begin to fail typically between 50 and 300 nm, that is, they peel off the substrate. For this reason, such layers did not find any practical use in spite of the high potential thereof.
As examples for the deposition of thick cubic boron nitride layers reference is made to the following publications identified at the end of the description:
[1] discloses cubic boric acid layers which are deposited via a plasma torch. With this method, however, only small surface areas of several mm2 can be coated. And even in this small coating area, the layer thickness distribution as well as the layer build up is, as a result of this method, extremely inhomogeneous. The layer thickness drops continuously down to zero and there are on the other hand coated areas in which the boron nitride is deposited almost exclusively in the non-cubic form. Furthermore, there is a limited reproducibility, which results in substantial variations in the growth rate. Concerning the coating of tools and machine components the required control of the plasma torch beam for complex tool and machine component geometries is extremely expensive and for this reason actually impossible to perform with high-quality resolution.
[2] discloses the generation of cubic boron nitride layers with a boron carbide target in an oxygen-free atmosphere. The layers which contain carbon but no oxygen however have a high internal tension level of up to 17 GPa.
[3] discloses the deposition of thick cubic boron nitride layers at high temperatures of typically 1200° C. Herein internal tensions in the layer are reduced bringing the silicon substrate to a plastic state. These measures for the reduction of the internal tensions and the high substrate temperatures however make the coating of tools impossible.
[4] discloses the manufacture of boron nitride layers without the addition of oxygen wherein the internal tensions are reduced by high-energy argon ion implantation at an energy level of 300,000 eV. By alternating cooling and ion implantation, which occurs in two different apparatus, cubic boron nitride layers with thicknesses of 1.3 μm could be produced. The area which is exposed is typically 1 cm2, the substrate volume is limited to 1 cm3 and the travel distance of the ions is about 180 nm. For the deposition of a 1.3 μm thick layer with an area of 1 cm2 seven cycles are needed including coating, ion-implantation and subsequent heat treatment and this treatment requires about half a month.
It is the object of the present invention to provide a layer composite wherein boron nitride of at least one individual layers is present in a cubic modification and the limitations and disadvantages mentioned above are overcome, particularly the concerning the coating of tools or machine components even in connection with layer thicknesses of more than 2 μm and relatively large lateral dimensions.